The mysterious case of overly active motion sensors

There is a problem with my MySensor network that has been annoying me for about a year now that I can't seem to solve. So here I am creating this thread, hoping for some help from you guys.
I have created several motion sensor nodes:

All of them are battery powered (tried 2aa, 3aaa for the molgan and CR2032) and all of them seemed to work fine at least for some time. Battery consumption was always pretty high though.

Then after different timeframes of about 6 weeks "disaster" struck every one of them: each one began to constantly trigger making them useless. First I though a low voltage level might be the problem but new batteries only worked for a short time (if at all) before seeing the same problem again.
My gateway is an arduino nano with a normal NRF24L01+ (+ a small cap) connected to it. Other sensors (a door contact switch and some temp/humidity sensors) work fine for the most part but there are some small hickups too (one isn't sending battery levels anymore although the other measurements still get send) and signing did not work on ~half of them. From what I gathered in this forum the connection did not work well enough for the longer messages.
I am planing to upgrade my gateway to use a PA LNA model with an antenna + external power supply and a nodeMCU for a MQTT gateway.
Would a bigger NRF only on the gateway side help? Any other ideas whats wrong with my network or why my motion sensors behave so strange?